The invention relates to a tunable oscillator circuit provided with a pair of oscillator transistors whose bases are connected crosswise to the opposite collectors, and a capacitance which is arranged between the emitters of the oscillator transistors, each emitter being connected to a reference voltage via an emitter current source.
A tunable oscillator circuit of this type--also referred to as a tunable astable multivibrator--is known per se, for example, from the article "Monolithic waveform generation" by A. B. Grebene, published in "IEEE Spectrum" of April 1972, pages 34-40. The known oscillator circuit is tuned by a variation of the emitter current sources of the oscillator transistors. An output is constituted by the collectors of the oscillator transistors. Each collector is connected to the supply voltage via a parallel arrangement of a collector resistor and a limiting diode. The limiting diodes determine and limit the capacitance voltage variation and, together with the magnitude of the currents of the emitter current sources and the magnitude of the capacitance C, they determine the frequency of oscillation. However, the tuning range at the high end is limited by parasitic capacitances of the oscillator transistors and the limiting diodes.